This invention relates generally to an exposure apparatus for projecting and printing a circuit pattern formed on a mask, onto a substrate being coated with a photosensitive material, in a reduced scale. More particularly, the invention is concerned with an exposure apparatus which uses deep ultraviolet light or an excimer laser as an exposure light source.
Reduction type projection exposure apparatuses are used in a process of manufacturing a semiconductor device which is formed with a very fine pattern such as LSI or VLSI. Miniaturization of a pattern has been required strongly due to increases in the integration density of a semiconductor device, and exposure apparatuses have been modified to meet such miniaturization, as well as improvements in a resist process.
The resolving power of an exposure apparatus can be improved by two methods, that is, a method in which the exposure wavelength is shortened, and a method in which the numerical aperture (NA) of a projection optical system is enlarged. Generally, the resolution is proportional to the exposure wavelength and it is inversely proportional to the NA. Besides the improvement of resolution, many attempts have been made to keep the depth of focus of a projection optical system. Generally, the depth of focus is proportional to the exposure wavelength, and it is inversely proportional to the square of the NA. Thus, improving the resolution and keeping the depth of focus are contradictory matters. As an attempt to solve such a problem, a phase shift method and a FLEX (Focus Latitude Enhancement Exposure) method, for example, have been proposed.
As regards the exposure wavelength, recently, KrF excimer lasers having an emission wavelength of about 248 nm are prevalently used in place of i-line of 365 nm. Also, ArF excimer lasers having an emission wavelength of about 193 nm are currently being developed, as a next generation exposure light source.
From the viewpoint of the production cost of a semiconductor device, further improvements in the throughput of an exposure apparatus have been attempted. For example, the power of an exposure light source is enlarged to thereby shorten the exposure time per one shot. Another example is enlarging the exposure area to thereby increase the number of chips per one shot.
In recent years, in order to meet the requirement of enlargement in chip size of a semiconductor device, the stream is shifting from step-and-repeat type exposure apparatuses (steppers) in which a mask pattern is printed sequentially in association with stepwise motion, to step-and-scan type exposure apparatuses in which a mask and a wafer are scanningly exposed in synchronism with each other, followed by stepwise motion to place a subsequent shot. In such step-and-scan type exposure apparatuses, the exposure field has a slit-like shape and, therefore, the exposure area can be enlarged without enlargement in size of the projection optical system.
Where ultraviolet light is used as an exposure light source, as described above, there may occur a phenomenon that, due to long-period use, ammonium sulfate (NH4) or silicon dioxide (SiO2) is deposited on the surface of an optical element disposed on the light path, to cause considerable degradation of the optical characteristic. The deposition is produced because of chemical reaction of ammonia (NH3), sulfurous acid (SO2) or silicon compound contained in the surrounding ambience caused in response to irradiation with ultraviolet light. In order to prevent such deterioration of optical elements, conventionally, the whole of the light path is purged by use of a clean dry air or an inert gas such as nitrogen.
As regards deep ultraviolet light, particularly, ArF excimer lasers having a wavelength of about 193 nm, it is known that there are plural absorbing bands for oxygen (O2) in the bandwidth about that wavelength. Also, ozone (O3) will be produced when oxygen absorbs light, and this ozone acts to increase light absorption, causing considerable decrease of transmission factor. Additionally, various products, as described above, attributable to the ozone will be deposited on the surface of an optical element, thus causing a decrease of the efficiency of the optical system.
In consideration of it, in an exposure optical system for projection exposure apparatuses having a deep ultraviolet light source such as an ArF excimer laser, for example, purge means using an inert gas such as nitrogen, for example, may be provided to keep the oxygen density along the light path at a low level.
An example of such inert gas purge means for an illumination optical system in a projection exposure apparatus, will be described with reference to FIG. 8.
As illustrated in the drawing, there are an excimer laser 201, and a container or housing 202 for the illumination optical system. Further, there are a reticle 203 and mirrors 204, 205 and 206. Denoted at 207 is a beam shaping optical system, and denoted at 208 is an optical integrator. Also, there are condenser lenses 209, 210 and 211.
A laser beam emitted by the excimer laser 201 is shaped by the beam shaping optical system 207 into a predetermined beam shape. Thereafter, the light enters the optical integrator 208 and, in response, secondary light sources (not shown) are produced near the light exit surface of the optical integrator 208. The light rays from the secondary light sources are directed through the condenser lenses 209, 210 and 211 to uniformly illuminate the reticle 203. Thus, the arrangement provides a Koehler illumination optical system.
In order to provide an inert gas ambience around the optical elements described above and along the light path of them, inert gas supply means (not shown) supplies a nitrogen gas, for example, into the housing 202 through a gas inlet port 202a. The thus applied inert gas flows through the interior of the illumination optical system. After substitution to remove any residual gas such as atmospheric gas, for example, the inert gas is discharged outwardly through a gas outlet port 202b, by gas discharging means (not shown).
The gas supply quantity may be controlled so as to minimize the substitution time by the inert gas, to thereby increase the system throughput, or minimize the consumption quantity of the inert gas after substitution, to thereby decrease the system running cost (Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994).
On the other hand, a currently prevailing illumination method is a variation illumination method (e.g., Japanese Laid-Open Patent Application, Laid Open No. 204114/1994) wherein the distribution of the secondary light source as described above is changed in various ways. This is to accomplish both a high resolution and a large depth of focus. In order that the illumination condition is made variable, many optical elements of an illumination optical system should be made interchangeable. With the above-described inert gas substitution method, on that occasion, it is very difficult to forcibly substitute the inside space of a mechanism (barrel) for holding optical elements to be interchanged. Particularly, in a case where an ArF excimer laser having an emission wavelength about 193 nm is used, there is a problem, as described, that the light absorption occurs due to any oxygen remaining along the light path which causes a serious decrease of optical efficiency. Therefore, forcible substitution of the interior of the movable barrel, if desired, needs a complicated structure for the gas flow passageway, and it causes an increase of the system cost as well as prolongation of the time for completion of the substitution which results in a decrease of the system throughput.
It is accordingly an object of the present invention to accomplish reduction of a substitution time to an inert gas ambience along an exposure light path, still with a minimum cost, and thereby to increase the system throughput.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: a light source; at least one (one or two or more) housing for accommodating therein an optical element disposed along an exposure light path extending from said light source to a substrate; first substitution means for substituting the interior of said housing with an inert gas ambience; and second substitution means for substituting the interior of a holding mechanism for holding the optical element accommodated in said housing, with an inert gas ambience.
The second substitution means may preferably include control means for controlling an inert gas supply quantity in accordance with the state of substitution of the inert gas ambience inside said holding mechanism and the state of substitution of the inert gas ambience inside said housing.
Each of the first and second substitution means may include control means for controlling an inert gas supply quantity, each control means being operable independently to set an inert gas supply quantity and a control operation timing.
The holding mechanism may comprise a barrel for movably holding said optical element in said housing.
The housing may accommodate therein the whole of or a portion of an illumination optical system for directing light from said light source to a reticle, and the holding mechanism may movably hold an optical element which may serve to variably or interchangeably set an illumination condition of said illumination optical system.
The housing may comprise a barrel for a projection optical system, and the holding mechanism may movably hold a lens inside said projection optical system, for variably or interchangeably setting an optical characteristic of said projection optical system.
The light source may comprise a light source of one of deep ultraviolet light and an excimer laser.
In accordance with another aspect of the present invention, there is provided a device manufacturing method including a process for producing a device by use of an exposure apparatus as recited above.
In accordance with a further aspect of the present invention, there is provided an exposure method, comprising the steps of: preparing at least one housing for accommodating therein an optical element disposed along an exposure light path extending from a light source to a substrate; substituting, by use of first substitution means, the interior of the housing with an inert gas ambience; and substituting, by use of second substitution means, the interior of a holding mechanism for holding the optical element accommodated in the housing, with an inert gas ambience, whereby the inside of the housing is substituted with an inert gas ambience.
In the exposure method described above, the second substitution means may be used to control an inert gas supply quantity in accordance with the state of substitution of the inert gas ambience inside the holding mechanism and the state of substitution of the inert gas ambience inside the housing.
Alternatively, each of the first and second substitution means may include control means for controlling an inert gas supply quantity, each control means being operable independently to set an inert gas supply quantity and a control operation timing.
Further, the holding mechanism may comprise a barrel for movably holding the optical element in the housing.
In the exposure method described above, the housing may accommodate therein the whole of or a portion of an illumination optical system for directing light from the light source to a reticle, and the holding mechanism may movably hold an optical element which may serve to variably or interchangeably set an illumination condition of the illumination optical system.
In the exposure method described above, the housing may comprise a barrel for a projection optical system, and wherein the holding mechanism movably holds a lens inside the projection optical system, for variably or interchangeably setting an optical characteristic of the projection optical system.
In the exposure method described above, the light source may comprise a light source of one of deep ultraviolet light and an excimer laser.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.